This invention relates to systems that use magnetic fields to determine an object""s location and orientation, and system gain factor.
As is known in the art, systems may use magnetic field measurements to indirectly determine the location and orientation of an object. These systems are useful, for example, in the medical field, because they are able to accurately locate an object in a patient""s body with only minimal intrusion into the body. The intrusion involves placing a small probe near the object to be located. The three-dimensional location and orientation of the probe is then determined from the effect that the probe""s location and orientation have on magnetic field measurements.
The probe may be either a source or a sensor of a magnetic field. If the probe is a source, sensors exterior to the body measure the field produced by the probe. If the probe is a sensor, magnetic sources exterior to the body produce the fields being measured.
Determining a probe""s location and orientation from magnetic field measurements is not straight forward because the measured magnetic fields are nonlinear functions of the location and orientation. To determine the probe""s location and orientation from the measured magnetic field values, the probe""s location and orientation are first presumed or xe2x80x9cguessedxe2x80x9d to be at a predicted location and orientation. An iterative process is used to compare values of the magnetic field at the guessed probe location and orientation with the measured field values. If the magnetic field values at a guessed location and orientation are close to the measured values, the guessed location and orientation are presumed to accurately represent the actual location and orientation of the probe.
The iterative process uses a physical model for the probe""s environment. The physical model specifies the location and orientation of each field source. From the specified locations and orientations, laws of electrodynamics determine the field values.
As the probe and its positioning system are physical systems, they are susceptible to various external influences (e.g., stray magnetic fields, field absorbing materials being positioned proximate the field generators and/or sensors, etc.) that affect the gain of the system. Additionally, these physical devices have various engineering tolerances (e.g., cable resistance, probe gain, input impedance, etc.) that also affect overall system gain. Accordingly, each time a component of the system is replaced, the system must be manually recalibrated.
According to an aspect of this invention, a system for determining the position, orientation and system gain factor of a probe includes magnetic field sources and at least one magnetic field sensor, such that a combination of a magnetic field sensor and a magnetic field source generates a unique measured magnetic field value. The system also includes a probe whose position and orientation affect the unique measured magnetic field values. A processor, coupled to receive these unique measured magnetic field values, iteratively processes measured magnetic field values to determine a system gain factor indicative of the gain of the probe and location factors indicative of the position and orientation of the probe. The number of unique measured magnetic field values generated must be at least equal to the sum of the number of factors calculated.
One or more of the following features may also be included. The iterative process is configured to determine a function of the differences between the measured magnetic field values and a plurality of predicted magnetic field values. The processor includes a calculated location process for calculating the predicted magnetic field values, in that the calculated location process guesses an initial gain, position and orientation for the probe, and then calculates the predicted magnetic field values based on a physical model and the initial gain, position and orientation. The initial position and orientation may be a predetermined or randomly selected fixed point.
The processor includes an optimization function for determining an extremum indicative of the differences between the measured magnetic field values and the predicted magnetic field values. The optimization function is a least squares sum function. The processor includes a repositioning process for adjusting the initial gain, position and orientation of the probe in response to the extremum being in a predefined range of unacceptable values, which is indicative of an unacceptable level of difference between the measured magnetic field values and the plurality of predicted magnetic field values. The location factors may include spatial, spherical, and/or rotational coordinates.
According to a further aspect of this invention, a method for determining the position, orientation and system gain factor of a three-dimensional object includes positioning a plurality of magnetic field sources proximate the three-dimensional object and positioning at least one magnetic field sensor in a fixed spatial relationship with that three-dimensional object. A combination of a magnetic field sensor and a magnetic field source generates a unique measured magnetic field value. Further, the position and orientation of the three-dimensional object affects these unique measured magnetic field values. The method determines a system gain factor indicative of the gain of the three-dimensional object and a plurality of location factors indicative of the position and orientation of the three-dimensional object. The number of unique measured magnetic field values generated must be at least equal to the sum of the number of factors calculated.
One or more of the following features may also be included. The step of determining a system gain factor and a plurality of location factors includes determining a function of the differences between the measured magnetic field values and a plurality of predicted magnetic field values. The step of determining a system gain factor and a plurality of location factors includes guessing an initial gain, position and orientation for the three-dimensional object and calculating the predicted magnetic field values based on a physical model and the initial gain, position and orientation. The step of determining a system gain factor and a plurality of location factors includes determining an extremum indicative of the differences between the measured magnetic field values and the predicted magnetic field values. The step of determining a system gain factor and a plurality of location factors includes adjusting the initial gain, position, and orientation of the three-dimensional object in response to the extremum being in a predefined range of unacceptable values, which is indicative of an unacceptable level of difference between the measured magnetic field values and the plurality of predicted magnetic field values.
While the system and method described above includes a plurality of magnetic field sources and at least one magnetic field sensor, the system can also include a plurality of magnetic field sensors and at least one magnetic field source. Further, while the system and method described above were said to include a probe, that probe can actually be any three-dimensional object, such as a hollow tube (e.g., a biopsy needle).
The advantages of the above aspects of the invention are numerous. As this system automatically calculates the system gain factor for the probe or three-dimensional object being utilized, device gain calibration is automated. Therefore, the need for tedious and time-consuming manual gain recalibration is eliminated. Additionally, as this calibration process is automated, the system components (e.g., probes, sensors, leads, etc.) can be quickly and easily swapped and the system reconfigured without the need for manual gain recalibration. This, in turn, streamlines and simplifies the reconfiguration process. Additionally, as the system automatically and continuously determines the system gain factor, the system can be physically moved without fear of external influences mandating manual gain recalibration.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.